Silicon nitride is one of the most promising high performance materials by virtue of its strength, its resistance to temperature changes and its corrosion resistance. It is used as heat resistant material in the construction of engines and turbines and in cutting tools.
Various processes for the preparation of silicon nitride by way of silicon diimide as intermediate stage have become known. They are based on the reaction of SiCl.sub.4 or SiS.sub.2 with ammonia at high or low temperatures and may be divided into four groups.
1. Reaction between a liquid phase containing SiCl.sub.4 and liquid ammonia at normal pressure or elevated pressure (U.S. Pat. No. 4,196,178).
2. Reaction between SiCl.sub.4 vapor and gaseous ammonia at elevated temperature (U.S. Pat. No. 4,145,224).
3. Reaction of SiS.sub.2 with liquid ammonia (M. Blix and W. Wirbelauer, Ber. Deut. Chem. Ges. 36, 4220 (1903)).
4. Reaction of liquid SiCl.sub.4 with solid ammonia (O. Glemser and E. Naumann in "Uber den thermischen Abbau von Siliciumdiimid", Z. Anorg. Allg. Chem. 289, page 134, (1959)).
The four processes mentioned here have significant defects which either lie in the process itself or lead to products having unsatisfactory properties. These defects are described in detail below.
The reaction of liquid SiCl.sub.4 with liquid ammonia is highly exothermic and therefore very difficult to control. NH.sub.4 Cl formed as by-product frequently blocks up the reaction apparatus.
In the process described in U.S. Pat. No. 4,196,178 one objective is to control the very vigorous reaction of SiCl.sub.4 with NH.sub.3 by dilution of the silicon tetrachloride with organic solvents. The organic solvents, however, introduce considerable quantities of carbon into the product, which seriously impair the capacity of the ceramic powder to sinter.
The second method described above, the reaction of gaseous SiCl.sub.4 and ammonia at about 1300.degree. C. to form Si.sub.3 N.sub.4, has the great advantage that, theoretically, the only by-product formed in addition to the desired product is hydrogen chloride, which should be very easy to remove from the solid product. In practice, however, silicon nitride prepared by this method still contains a considerable quantity of chemically bound chlorine, which can only be removed with great difficulty.
The silicon diimide which is prepared by the reaction of SiS.sub.2 with ammonia is also found to be insufficiently pure owing to significant proportions of sulphur remaining in the product.
The above-described process for the preparation of silicon diimide by the reaction of solid ammonia with liquid SiCl.sub.4 is disadvantageous in two respects: firstly, on account of the great technical expenditure required for this method of preparation; secondly, on account of the lack of reaction control due to the fact that the exothermic reaction of SiCl.sub.4 with ammonia causes the latter to melt.
U.S. Pat. No. 4,725,660 discloses a process by which Si(NHCH.sub.3).sub.4 is reacted with NH.sub.3 in the absence of solvents at temperatures above the melting point of the silane to form polysilazane having the composition Si(NH).sub.2. The product is obtained in the form of a resin. After it has been pyrolyzed, the product of pyrolysis still contains about 16% of carbon. In fact, therefore, only a polysilazane containing carbon can be obtained by this process owing to premature polymerization.
It is an object of the present invention to provide a silicon diimide which does not have the disadvantages described above and is suitable for further processing to high quality Si.sub.3 N.sub.4.